They diverged from each other one to 2 million years ago. Compared with their ancestors' split from humans about 6 million years ago, that's pretty recent. Even though they're closely related, their behavior is very different. Chimps are more aggressive, and use intimidation to interact with others. They are the warriors. Bonobos, on the other hand, are more tolerant and playful, and use sex to handle their anxiety. They are more aggressive than humans, but considered the "lovers" of the primate world. Scientists think those distinctions are caused by physical differences in the brain. Yes! They used neuro-imaging to measure how developed different brain regions are. Bonobos have more developed areas in the limbic system, the emotional part of the brain. Two of those areas, the amygdala and anterior insula are thought to be areas responsible for empathy in humans. They also found that the pathway connecting the amygdala to the prefrontal cortex is larger, giving bonobos better ability to sense another individual's stress. Chimps have better visual pathways, which would enable them to be better tool users.